In recent years, lithium ion secondary cells have been used as a power supply cell of portable electronics such as cell phones and laptops.
In general, a positive electrode of a lithium ion secondary cell is obtained by forming a mixture layer by applying a slurry to a collector and drying the collector with the slurry, the slurry being produced by adding a positive electrode material in which a powdery active material is dispersed in a binding agent to a solvent.
Conventionally, a lithium metal oxide such as a lithium cobalt oxide (LiCO2) has been used as an active material of a positive electrode (positive electrode active material). The lithium cobalt oxide is a material which can be expected to provide a high cell capacity, but has problems in that the cobalt in the lithium cobalt oxide is expensive and availability is poor.
Therefore, the application of a lithium-nickel-cobalt-manganese composite metal oxide has been recently considered as a positive electrode active material which can be expected to provide the same cell capacity as the lithium cobalt oxide.
In addition, a positive electrode material may contain a conductive assistant such as conductive carbon to sufficiently interchange electrons between a positive electrode active material and a collector by increasing electron conductivity of the electrode.
PATENT DOCUMENT 1 discloses a core-shell-type positive electrode active material formed of a core portion made from a lithium metal oxide and a shell portion which is formed by coating the core portion with a material containing barium titanate, a metal oxide, and a conductive assistant. According to PATENT DOCUMENT 1, by coating the lithium metal oxide with the barium titanate and the metal oxide, electric resistance is increased and thermal stability of the positive electrode active material is improved. Furthermore, since the shell portion forming material contains the conductive assistant, excellent electric discharge characteristics of the positive electrode active material can be maintained.